Cellulosic fibrous structures, such as paper webs, are well known in the art. Low-density fibrous webs are in common use today for paper towels, toilet tissue, facial tissue, napkins, wet wipes, and the like. The large consumption of such paper products has created a demand for improved versions of the products and the methods of their manufacture. In order to meet such demands, papermaking manufacturers must balance the costs of machinery and resources with the total cost of delivering the products to the consumer.
Various natural fibers, including cellulosic fibers, as well as a variety of synthetic fibers, have been employed in papermaking. Typical tissue paper is comprised predominantly of cellulosic fibers. The overwhelming majority of the cellulosic fibers used in tissue are derived from trees. Many species are used, including long fiber containing softwoods (conifer or gymnosperms) and short fiber containing hardwoods (deciduous or angiosperms). In addition, many different pulping approaches may be used. On one hand, there are Kraft and sulfite pulping processes followed by intense bleaching that produce flexible, lignin-free and very white fibers. On the other hand, there are thermo-mechanical or chemi-mechanical pulping processes that produce higher lignin containing fibers that are less flexible, prone to yellowing in sunlight and poorly wettable. As a general rule, the more lignin the fibers contain the less expensive they are.
Despite the broad range of fibers used in papermaking, cellulose fibers derived from trees are limiting when used exclusively in disposable tissue and towel products. Wood fibers are generally high in dry modulus and relatively large in diameter, which causes their flexural rigidity to be high. Such high-rigidity fibers tend to produce stiff non-soft tissue. In addition, wood fibers have the undesirable characteristic of having high stiffness when dry, which typically causes poor softness of the resulting product, and low stiffness when wet due to hydration, which typically causes poor absorbency of the resulting product. Wood-based fibers are also limiting because the geometry or morphology of the fibers cannot be “engineered” to any great extent. Except for relatively minor species variation, papermakers must accept what nature provides.
To form a useable web, the fibers in typical disposable tissue and towel products are bonded to one another through chemical interaction. If wet strength is not required, the bonding is commonly limited to the naturally occurring hydrogen bonding between hydroxyl groups on the cellulose molecules. If temporary or permanent wet strength is required in the final product, strengthening resins can be added. These resins work by either covalently reacting with the cellulose or by forming protective molecular films around the existing hydrogen bonds. In any event, all of these bonding mechanisms are limiting. They tend to produce rigid and inelastic bonds, which detrimentally affect softness and energy absorption properties of the products.
The use of synthetic fibers that have the capability to thermally fuse to one another and/or to cellulose fibers is an excellent way to overcome the previously mentioned limitations. Wood-based cellulose fibers are not thermoplastic and hence cannot thermally bond to other fibers. Synthetic thermoplastic polymers can be spun to very small fiber diameters and are generally lower in modulus than cellulose. This results in the fibers' very low flexural rigidity, which facilitates good product softness. In addition, functional cross-sections of the synthetic fibers can be micro-engineered during the spinning process. Synthetic fibers also have the desirable characteristic of water-stable modulus. Unlike cellulose fibers, properly designed synthetic fibers do not lose any appreciable modulus when wetted, and hence webs made with such fibers resist collapse during absorbency tasks. The use of thermally bonded synthetic fibers in tissue products results in a strong network of highly flexible fibers (which is good for softness) joined with water-resistant high-stretch bonds (which is good for softness and wet strength).
Accordingly, the present invention is directed to fibrous structures comprising cellulosic and synthetic fibers in combination, and processes for making such fibrous structures.